The applicants have shown that Electron Spin Resonance (ESR) spectroscopy can directly detect free radical generation in isolated perfused rat and rabbit hearts. However, it is unknown whether free radicals are generated during cardiac operations. They hypothesize that free radicals are manifest intraoperatively in myocardial tissue during the cycle of ischemia and reperfusion. They plan to investigate clinically relevant aspects of the free radical hypotheses relating to the myocardial injury in isolated, perfused rat and rabbit hearts subjected to conditions of surgical ischemia and reperfusion. They will attempt to directly detect and rigorously characterize free radicals produced in the cardiac surgical model by the combined use of ESR trapping and Mass Spectrometry. They will also explore and advance the possibility of in vivo ESR measurements of free radicals in intact, isolated hearts subjected to normothermic and hypothermic ischemia. They will determine (Specific Aim 1) whether surgical ischemia and reperfusion results in the generation of toxic radical species. They will then determine (Specific Aim 2) possible mechanisms of radical generation during surgical ischemia and reperfusion, and then assess (Specific Aim 3) the extent to which free radical generating processes are operative during clinically relevant conditions of hypothermic ischemia in comparison to normothermic ischemia. Finally, they will assess (Specific Aims 4 and 5) the feasibility of directly measuring free radicals and the concentration of molecular oxygen in intact hearts subjected to normothermic and hypothermic ischemia.